The invention relates to a novel process for the preparation of phloroglucinol (1,3,5-trihydroxybenzene) from benzene tricarboxylic acid-(1,3,5)-triamide.
Several snytheses of phloroglucinol are known. Especially important from a technical standpoint is the reduction of 1,3,5-trinitrobenzene to 1,3,5-triaminobenzene and subsequent hydrolysis of the latter. According to earlier processes, the reduction can be carried out with tin in hydrochloric acid solution (Weidel and Pollak, Monatsh. 21,15, (1900); Hepp, Ann. 215, 348; Organic Synthesis Coll. Vol. I, 444 (1932); US-P 2 461 498) or with hydrogen and Raney nickel in an organic solvent, especially ethylacetate (WG-P 813 709; Gill et al., J. Chem. Soc., 1753 (1949); GB P 1 106 088). Iron/hydrochloric acid is a suitable reduction agent for large-scale reduction of trinitrobenzene (US P 2 614 126; Kastens, Ind. and Engin. Chem. 42, 402 (1950); GB P 1 022 733). Platinum, palladium and rhodium catalysts have also been suggested for the reduction of trinitrobenzene (FR. P 1 289 647; Desseigne, Mem. Poudras 44, 325 (1962). For this synthesis instead of 1,3,5 trinitrobenzene one can also use 2,4,6.-trinitrobenzoic acid, which on a large-scale can be obtained by oxidation of trinitrotoluene with sodium dichromate in sulfuric cid (Kastens, 1.c.), since the 2,4,6-triaminobenzoic acid obtained in the reduction is either immediately decarboxylated to triaminobenzene or is converted to phloroglucinol in the subsequent hydrolysis (GB P 1 022 733; GB P 1 106 088; GB P 1 274 551). It is furthermore known to use 5-nitro-1,3-diaminobenzene insteand of trinitrobenzene (GB P 1 012 782). Hydrolysis of triamine to phloroglucinol is normally carried out in an inorganic acid solution (Flesch, Monatsh. 18, 755 (1897); WG P 102 358), and according to a more recent process in the presence of copper and/or its salts as catalyst (WG P 1 195 327).
Phloroglucinol can also be prepared by an interesting large-scale process by oxidation of 1,3,5-triisopropylbenzene, separation of trihydroperoxide from the mixture of mono-, di- and trihydroperoxide with subsequent ketonic cleavage of the latter (GB P 751 598; EG P 12 239; Seidel et al; Journ. prakt. Chemie 275, 278 (1956). It is also possible to oxidize triisopropylbenzene with oxygen in acetic acid anhydride directly to phloroglucinol triacetate and to hydrolyze the latter with alcoholic sodium hydroxide to phloroglucinol (US P 2 799 698). One can also start from m-isopropylresorcinol, esterifying the latter with acetic acid annhydride, oxidizing the resulting m-isopropylresorcinol-diacetate to hydroperoxide and finally react the latter with acid to phloroglucinol (US P 3 028 410).
Phloroglucinol can furthermore be obtained by melting resorcinol (Barth and Schreder, Ber. 12, 503 (1879), chlorine or bromine substituted resorcinol in the 2-,4-, 5-, 3,5- or 2,4-position (WG P 2 231 005) or 1,3,5-benzenetrisulfonic acid (US P 2 773 908) with excess alkali hydroxide.
Apart from the cited benzene derivatives, hexoxybenzene, picryl chloride, tetrachlorobenzene and tetrabromobenzene as well as tribromobenzene, were mentioned an initial materials for the synthesis of phloroglucinol; hexaoxybenzene is hydrated in aqueous medium with platinum oxide (Kuhn et al., Ann. 565, 1 (1949), picryl chloride is reduced with tin and hydrochloric acid or electrolytically with subsequent hydrolyzation of the resulting l,3,5-triaminobenzene or 2,4,6-triamino-1-chlorobenzene (Heertjes, Recueil 78, 452 (1959). The cited tetrahalobenzenes are ammonolyzed in the presence of a copper catalyst and the intermediate triamine is hydrolyzed without prior separation in the reaction mix (US P 3 230 266). Tribromobenzene can be reacted with sodium methanolate and catalytic amounts of Cu iodide in methanol/dimethylformamide as solvent to 1,3,5-trimethoxybenzene, which is finally also hydrolyzed (McKillop et al., Synthetic Communications 4(1).fwdarw., 35 (1974).
It is also known to synthesize phloroglucinol using diethyl malonate as starting material; when treated with metallic sodium there is a spontaneous condensation of diethyl malonate to the trisodium salt of phloroglucinol-di-carboxylic acid diethylester, this intermediate product is subsequently subjected to alkaline hydrolysis and decarboxylation (v. Baeyer, Ber. 18, 3454 (1885); Willstaetter, Ber. 32, 1272 (1899); Leuchs, Ber. 41, 3172 (1908); Kominos, Bull. Soc. Chim. Fr. 23, 449 (1918). This synthesis was improved to the extent that sodium diethyl malonate and trisodium salt of phloroglucinol dicarboxylic acid diethylester is obtained in a single step by boiling in a neutral solvent of high BP, preferably Dekalin (EG P 24 998).
Among the above-mentioned processes, only the process based on 2,4,6 trinitrobenzoic acid ester, has obviously so far found acceptance in the art. This process has, however, a number of serious drawbacks. 2,4,6-trinitrobenzoic acid is obtained by oxidation of explosive trinitrotoluene, hence the process is dangerous. Moreover, the total yield, based on 2,4,6-trinitrobenzene via trinitrobenzene, triaminobenzene to phloroglucinol, is very small. The process also has drawbacks because of the waste water problem: the waste water of the oxidation and reduction reactions is highly acid, contains heavy metal chromium and/or iron and requires processing.